check type rather than value

scss/mixins/_breakpoints.scss

@@ -16,7 +16,7 @@
 //    md
 @function breakpoint-next($name, $breakpoints: $grid-breakpoints, $breakpoint-names: map-keys($breakpoints)) {
   $n: index($breakpoint-names, $name);
-  @if $n == null {
+  @if type-of($n) == null {
     @error "breakpoint `#{$name}` not found in `#{$breakpoints}`";
   }
   @return if($n < length($breakpoint-names), nth($breakpoint-names, $n + 1), null);